Induction motors have been used as driving sources of compressors that are mounted in electronic refrigerators, air conditioners, and the like and that operate at constant speed and thereby do not require speed control. The induction motor is advantageous in that it has a robust structure. Furthermore, the induction motor can be structured inexpensively because it can be directly initiated with commercial power. As needs for high efficiency have been increasingly raised, there is a desire for development of self-initiated permanent magnet synchronous motors that can be initiated by itself with commercial power and can achieve high-efficiency operation.
The self-initiated permanent magnet synchronous motor has cage-type conductors used for initiation along the outer rotor periphery. Permanent magnets need to be disposed along the inner periphery of the cage-type conductors, imposing a limit on a space in which the magnets are disposed. Methods of increasing the efficiency of this type of motor and obtaining high torque from it are disclosed in, for example, Patent Document 1 (Japanese patent laid-open No. 2002-233087) and Patent Document 2 (Japanese patent laid-open No. 2005-117771). In each method, arrangement of permanent magnets is optimized in a limited space.
From the viewpoint of direct driving with commercial power, a power factor is also an important design target. The power factor is an index indicating how efficiently power supplied from the electric power company is used; the higher the power factor of a unit is, the more efficiently the unit uses power generated by the electric power company. Major electric power companies give a discount when the power factor is 85% or more, and charges an extra fee when the power factor is less than 85%. Accordingly, whether the power factor is at least 85% is a very important guide in a phase in which a self-initiated permanent magnet synchronous motor is designed. However, this is not addressed in the Patent Documents 1 and 2.